Unlike wireline communications, the wireless environment accommodates essentially an unlimited number of users sharing different parts of the frequency spectrum and very strong signals coexist next to very weak signals. A radio receiver must be able to select the signal of interest, while rejecting all others.
Among the important problems faced by the designers of radio receivers are image rejection and monolithic integration. A radio receiver must be able to select the desired signal from its image. Otherwise, the subsequent detector circuit will be unable to distinguish between the desired signal and the image signal and, therefore, the output will be the result of the superposition of both. As wireless communications units evolve, means to reduce cost, size, and weight through monolithic integration are critical.
Image signal rejection relates to the ability of the radio frequency receiver to select the desired signal from the image signal of the desired signal spaced away by twice the intermediate frequency signal. This is important as the subsequent detector circuit will be unable to distinguish between the desired signals and the image signals and, therefore, the output of the detector circuit will be the result of the superposition of both. This is the essence of the image signal problem.
In conventional heterodyne receiver architectures, a large and expensive ceramic or Surface Acoustic Wave (SAW) filter is positioned between the low noise amplifier and the mixer to suppress the image signal. This arrangement is attractive in terms of current consumption. The arrangement defies integration, however, and results in excessive size, weight, and cost.
There also have been efforts to use phasing methods to achieve image signal rejection in the mixer itself. U.S. Pat. No. 5,870,670 and U.S. Pat. No. 5,678,220 provide examples of such efforts. Image reject mixers in which phasing methods are used are at best, however, only capable of achieving 30 dB of image rejection over the typical temperatures and processes used. The limitation, in terms of reliable image rejection from the phasing methods, comes from the required amplitude and phase imbalance in the local oscillator quadrature generation and intermediate frequency quadrature combining. It can be shown mathematically that achieving even 30 dB of image rejection using the phasing method requires less than 1° and 0.5 dB of phase and amplitude balance, respectively. The phasing methods of achieving image rejection, while improvements in terms of integration and cost, require additional filtering to meet overall system image rejection.
Other attempts at image rejection have involved image “traps” in the form of a simple series inductance-capacitance (or “L-C”) circuit across the differential line. This approach results in an excess inductance in the desired band that must be tuned out. Traditionally, a series capacitor has been used to tune out the in-band inductance. This approach suffers, however, from the fact that an additional mixer DC return is required. An on-chip choke, to provide this DC return, would be large and have considerable DC resistance. The increased space requirements add expense and the increased DC resistance in the ground return path lowers the voltage headroom on the mixer limiting its dynamic range.
U.S. Pat. No. 5,630,225 describes an arrangement by which a dielectric member is placed in proximity to a transmission line. The electromagnetic properties of this member alter the frequency response characteristic of the system by the formation of a notch at the image signal frequency. Such an arrangement is not amenable to monolithic integration. The dielectric member does not have the requisite electrical characteristics for such an application and the physical size of the dielectric member makes it unsuitable for monolithic integration.
FIG. 1 is a schematic drawing of a portion of a prior art radio frequency receiver. In FIG. 1, a radio frequency signal is received by a low noise amplifier 10 and, after amplification, is supplied to an image trap filter 12 that filters the image signal from the amplified radio frequency signal. The radio frequency signal then is supplied to a mixer 14 that develops the intermediate frequency signal from the radio frequency signal.
In modern radio frequency receivers for wireless applications, typically 50 dB of image signal filtering is required from the overall system. This image signal filtering comes from a combination of pre-select band pass filtering, image filtering and possible use of an image reject mixer. This high image signal rejection requirement means that the contribution of each portion of the receiver, where image signal rejection takes place, to the overall image signal rejection is critical.
In addition, in a multi-band radio frequency receiver, the image signal filter must pass all the radio frequency signal bands and must reject all the image signals simultaneously. A fundamental problem in designing an image signal filter for a multi-band radio frequency receiver is that a higher order image signal filter (i.e., a higher number of poles in the filter response) is required. This is illustrated by FIG. 2.
Furthermore, problems arise when there is degradation in image signal rejection in an image trap filter due to process variations (i.e., variations in the values of components, such as resistors, capacitors and inductors, in the image trap filter). The possible variations of component values used to construct a monolithic filter requires significant margins in the image signal rejection response of an image signal filter.